I installed High Sierra and now my iMac (27" 2011) can not see the external USB TimeMachine drive.

The drive shows up in DiskUtil but it's greyed out. I can not mount it. In fact it doesn't respond to any commands, mount, eject, first aid won't run.

I also have a MacBook Pro (High Sierra too), it can't read the drive, same problem.

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I tried booting into Recovery mode, I still can't access the drive.

The drive is fine, I can plug it into a Linux machine and I can browse the disc.

How can I fix this?

Thanks Rich

1 Answer 1


Ok something is broken in High Sierra disk utils (as of 2018/04/02). It seems unable to mount discs correctly. And I think crashes. I suspect it's something to do with the EFI partition but I'm only guessing.

To fix my problem I had to format the drive and start the backup from scratch. This did fix the problem.

However you can not format the disc from disk tools, it crashes. You have to format it via the terminal.

I followed this short guide.


I then ran Time Machine and selected the new disc.

Not quite sure how a bug like this manages to fly under the radar at Apple. Poor show.

  • Did you do a fsck from Terminal on the drive before wiping it? Often if it won't mount, there's something happening during the pre-mount filesystem check that causes that to fail. Might have pointed to something solvable without a wipe. Linux doesn't necessarily perform the same checks and fail to mount based on their outcome.
    – Sera H
    Feb 15, 2018 at 10:11
  • Ah ok, no I didn't do that. Can you fix the problem from there? Or would I have still had to wipe it?
    – Rich
    Feb 15, 2018 at 12:06
  • Depends on what the issue was. Disk Utility is basically just a GUI for diskutil but often times the command line version is more helpful in telling you what's going on (it won't just crash and disappear, for instance).
    – Sera H
    Feb 15, 2018 at 23:17
  • I did try command line diskutil but I couldn't get it to mount that way either. Also my mac book (upgraded to high sierra) couldn't mount the drive either. Make me think it's an OS thing.
    – Rich
    Feb 16, 2018 at 10:21

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